This is a nationalization under 35 U.S.C. § 371 of International Application PCT/GB02/02774, filed under the Patent Cooperation Treaty on Jun. 17, 2002, claiming priority to Application Serial No. 0114744.6, filed in Great Britain on Jun. 16, 2001. Accordingly, we claim priority under 35 U.S.C. § 119 to Application Serial No. 0114744.6, filed in Great Britain on Jun. 16, 2001.
1. Field of the Invention
The present invention relates to a method of processing data, in particular to the processing of seismic data acquired using single sensor seismic acquisition.
2. Description of the Related Art
A conventional method of seismic data acquisition is illustrated schematically in FIG. 1. In this conventional method, seismic data are acquired by an array 1 containing a plurality of sensors 2, 2′. The sensors 2, 2′ in FIG. 1 are arranged in a linear array, but they could alternatively be arranged in, for example, a two-dimensional array. In the linear array 1 of sensors shown in FIG. 1, each sensor is separated from adjacent sensors by a substantially constant distance. The distance d between the centre of one sensor and the centre of an adjacent sensor is approximately 3.125 m in this example, although conventional embodiments of seismic acquisition systems have widely varying sensor separations.
A typical array of seismic sensors contains a large number of sensors. It is conventional practice for the sensors in an array to be “hard-wired” into groups of near-by sensors, where each sensor in a group receives substantially the same signal component from the sub-surface target to be imaged. Grouping is intended to improve the signal-to-noise ratio by electrically merging the analogue signals from each sensor into a single signal. This single signal represents an estimate of the signal which is common to all the sensors in the group, and is generally known as a “common signal”.
The group length (that is, the length of a group of sensors) is chosen such that the coherent noise to be suppressed by the analogue merging has spatial wavelengths which lie between twice the sensor spacing and the group length. Random noise is assumed to be uncorrelated from sensor to sensor and thereby attenuated by the merging process. The group length and sensor spacings are largely fixed for the duration of a survey. The hard-wiring of sensors into groups is done before the grouped output signal is digitised for transmission to the recording system.
In the example shown in FIG. 1, the sensors are hard-wired into groups, with each group containing nine adjacent sensors. The extent of each group is therefore approximately 25 m. In the array of FIG. 1, adjacent groups “overlap” with one-another, as the sensor 2′ is both the right-handmost sensor of the jth group and the left-handmost sensor of the kth group. Other amounts of overlap may be used in different acquisitions, with a goal of ensuring that the seismic wavefield reflected from the sub-surface target is adequately sampled in space to capture the spatial variations of the target to be imaged and to ensure that there is no leakage of spatially-aliased energy. This commonly results in the group length being twice the group interval (50% overlap), but the overlap principle is sufficiently illustrated by the simple one sensor overlap in FIG. 1.
The outputs of the individual sensors in each group are merged in a pre-determined manner, and the merged output of each group is used for further signal processing. Thus, the output of the seismic data acquisition arrangement of FIG. 1 consists of a series of merged outputs, one merged output for each group. FIG. 1 shows the merged output from the jth group, as {overscore (S)}j, and the merged output from the kth group, as {overscore (S)}k. The conventional process of merging the individual outputs of the sensors in each group is generally an ensemble averaging process, that yields a weighted average of the individual sensor outputs representing the estimate of the signal common to all the sensors in the group.
Single sensor seismic (SSS) is a new concept in the acquisition of seismic data and is embodied in Schlumberger's Q system which is described by J Martin et al in “Acquisition of marine point receiver seismic data with a towed streamer”, Expanded Abstract ACQ 3.3, 60th Annual International Meeting of the Society of Exploration Geophysicists, Calgary (2000), by G. Baeten et al in “Acquisition and processing of point receiver measurements in land seismic”, Expanded Abstract ACQ 3.4, 60th Annual International Meeting of the Society of Exploration Geophysicists, Calgary (2000) and by G. Baeten et al in “Acquisition and processing of point source measurements in land seismic”, Expanded Abstract ACQ 3.5, 60th Annual International Meeting of the Society of Exploration Geophysicists, Calgary (2000). SSS data acquisition is distinguished from the conventional seismic data acquisition in that the individual output of each sensor is available for signal processing operations. In SSS data acquisition, the seismic wave field is sampled by sensors, each of which produces an individual digital output signal. The digital output signal for each sensor is available for further signal processing operations. SSS data acquisition can be used with seismic sensors disposed on land, at or near the sea surface, within the water column, within a bore hole, on the sea bed, or buried into the sea bed. The sensors may be single or multiple component sensors sampling pressure, displacement, velocity, acceleration or pressure gradient, or combinations thereof.